Traditional chemotherapy is often accompanied by systemic toxicity to the patient. Monoclonal antibody which can offer an alternative tumor-selective treatment approach via discriminating antigens on cancer cells, is normally not sufficiently potent to be therapeutically active by itself. Antibody-drug conjugates (ADCs), which by their name have the exquisite targeting ability of antibodies in combination with the cytotoxic action of anticancer agents, enable to target and deliver drugs to cancer cells leaving normal cells largely unaffected, thus improving the therapeutic index of chemotherapeutic agents. since US FDA approvals of Adcetris (brentuximab vedotin) in 2011 and Kadcyla (adotrastuzumab emtansine) in 2013, the applications of antibody-drug conjugate (ADC) as a promise targeted treatment of cancers have been exploded and almost every major pharmaceutical and biotech company has adopted this approach (Chari, R. et al, Angew. Chem., Int. Ed. 2014, 53, 3796-3827; Sievers, E. L. et al. Annu Rev Med. 2013, 64, 15-29; Mehrling, T. Future Oncol, 2015, 11, 549). Antibody-drug conjugates are now a drug class with a robust pipeline and a flurry of deal-making among pharmaceutical companies. With more than 50 ADCs currently in various stages of clinical trials according to www.clinictrails.gov, and even more in preclinical and/or are ready to enter first-in-human trials, the market is rife with eager anticipation of more ADC drugs to be approved by regulators.
The first-generation ADCs, including Kadcyla and Adcetris, are produced through nonselective conjugation of native lysine amines or interchain cysteine thiols on an antibody respectively to a cytotoxic drug. Since there are over 50 surface-exposed lysines and 8 hinge cysteine residues in IgG1 antibodies, this nonselective conjugation results in randomly cross-linkage of cytotoxic drugs to practically all areas of the antibody molecule, particularly having a diverse population of ADCs with a wide distribution of drugs per antibody (DAR) (Wang, L., et al. 2005 Protein Sci. 14, 2436; Hamblett, K. J., et al. 2004 Clin. Cancer Res. 10, 7063). Thus some of the undesired ADC subpopulation could lead to shorter circulation half-life, lower efficacy, potentially increased off-target toxicity and a wide range of in vivo pharmacokinetic (PK) properties (Hamblett, K. J. et al, Clin. Cancer Res. 2004, 10, 7063-7070; Adem, Y. T. et al, Bioconjugate Chem. 2014, 25, 656-664; Boylan, N. J. Bioconjugate Chem., 2013, 24, 1008-1016; Strop, P., et al 2013 Chem. Biol. 20, 161-167). In addition, with this classical conjugation, the batch-to-batch consistency in ADC production can be challenging and may require diligent manufacturing capabilities (Wakankar, A. mAbs, 2011, 3, 161-172).
Therefore, biotechnology companies and academic institutions are intensely focusing on establishing novel reliable methods for site-specific ADC conjugation. So far, there are several approaches developed in recent years for site selective ADC preparation (Panowski, S, 2014, mAbs 6, 34). They include incorporation of unpaired cysteines, e.g. engineered reactive cysteine residues, called THIOMAB from Genentech (Junutula, J. R., et al 2010 Clin. Cancer Res. 16, 4769; Junutula, J. R., et al 2008 Nat Biotechnol. 26, 925-32; U.S. Pat. Nos. 8,309,300; 7,855,275; 7,521,541; 7,723,485, WO2008/141044), genetically introduced glutamine tag with Streptoverticillium mobaraense transglutaminase (mTG) (Strop, P., Bioconjugate Chem., 2014, 25, 855-862; Strop, P., et al., 2013, Chem. Biol. 20, 161-167; U.S. Pat. No. 8,871,908 for Rinat-Pfizer) or with Microbial transglutaminase (MTGase) (Dennler, P., et al, 2014, Bioconjug. Chem. 25, 569-578. US pat appl 20130189287 for Innate Pharma; U.S. Pat. No. 7,893,019 for Bio-Ker S.r.l. (IT)), incorporation of thiolfucose (Dennler, P., et al, 2014 Bioconjugate Chemistry 25, 569; Okeley, N. M., et al 2013 Bioconjugate Chem. 24, 1650), incorporation of unnatural amino acids through mutagenesis (Axup, J. Y., et al., 2012, Proc. Natl. Acad. Sci. 109, 16101-16106; Zimmerman, E. S., et al., 2014, Bioconjug. Chem. 25, 351-361; Wu, P., et al, 2009 Proc. Natl. Acad. Sci. 106, 3000-3005; Rabuka, D., et al, 2012 Nat. Protoc. 7, 1052-67; U.S. Pat. No. 8,778,631 and US Pat Appl. 20100184135, WO2010/081110 for Sutro Biopharma; WO2006/069246, 2007/059312, U.S. Pat. Nos. 7,332,571, 7,696,312, and 7,638,299 for Ambrx; WO2007/130453, U.S. Pat. Nos. 7,632,492 and 7,829,659 for Allozyne), Incorporation of selenocysteine into antibodies (Hofer, T., et al 2009, Biochemistry 48, 12047-12057; U.S. Pat. No. 8,916,159 for US National Cancer Institute), Convertion of cysteines located in the CXPXR consensus sequence to formylglycine (FGly) with formylglycine generating enzyme (FGE) (Drake, P. M., et al., 2014, Bioconjug. Chem. 25, 1331-1341. Carrico; Isaac S. et al U.S. Pat. Nos. 7,985,783; 8,097,701; 8,349,910, and US Pat Appl 20140141025, 20100210543 for Redwood Bioscience), and through glycoengineeringly introduction of sialic acid with the use of galactosyl- and sialytransferases (Zhou, Q., et al 2014, Bioconjug. Chem., 25, 510-520, US Pat Appl 20140294867 for Sanofi-Genzyme). These above methods have produced nearly homogeneous product profiles, but they are required antibody-engineering processes and reoptimization of cell culture conditions. Moreover, expression yields for genetic encoding of an unnatural amino acid were typically not promisingly high enough (Tian, F., et al, 2014, Proc. Natl. Acad. Sci. U.S.A. 111, 1766-71) which has a significant impact on the cost of goods of the ADC. In addition, it has been known that ADCs obtained by conjugation to cysteine side chains often display limited stability in circulation, leading to premature disconnection of the cytotoxic payload before the tumor site is reached (Junutula, J. R., et al 2008, Nat. Biotechnol. 26, 925-32).
The disulfide bond structures of the four subclasses of IgG antibodies were known in the 1960s (Milstein C. Biochem J 1966, 101:338-351; Pink J R, Milstein C. Nature 1967, 214:92-94; Frangione B, Milstein C. Nature 1967, 216:939-941; Pink J R, Milstein C. Nature 1967, 216:941-942; Frangione B, et al. Biochem J 1968, 106, 15-21; Frangione B, Milstein C. J Mol Biol 1968; 33:893-906; Edelman G M, et al. Proc Natl Acad Sci USA 1969; 63:78-85; Frangione B, et al. Nature 196, 221:145-148, Spiegelberg, H. L. et al Biochemistry, 1975, 10, 2157-63). Disulfide bond structure is critical for the structure, stability, and biological functions of IgG molecules. Among the four subclasses of IgG antibodies, IgG1, IgG2, IgG3 and IgG4, each IgG contains a total of 12 intra-chain disulfide bonds; each disulfide bond is associated with an individual IgG domain. The two heavy chains are connected in the hinge region by a variable number of disulfide bonds: 2 for IgG1 and IgG4, 4 for IgG2 and 11 for IgG3. The light chain of the IgG1 is connected to the heavy chain by a disulfide bond between the last cysteine residue of the light chain and the fifth cysteine residue of the heavy chain. But, for IgG2, IgG3 and IgG4, the light chain is linked to the heavy chain by a disulfide bond between the last cysteine residue of the light chain and the third cysteine residue of the heavy chain (Liu, H. and May, K., 2012, mAbs 4, 17-23). On the ranks of the susceptibility of disulfide bonds in human IgG1 antibodies by experimental reduction, differential alkylation, and LC-MS analysis (Liu, H, et al Anal. Chem., 2010, 82, 5219-5226), inter chain disulfide bonds are more susceptible to reduction than intra chain disulfide bonds, and the disulfide bonds between the light chain and heavy chain were more susceptible than disulfide bonds between the two heavy chains. The upper disulfide bond of the two inter heavy chain disulfide bonds was more susceptible than the lower one. Furthermore, disulfide bonds in the CH2 domain were the most susceptible to reduction. Disulfide bonds in VL, CL, VH, and CH1 domains had similar and moderate susceptibility, while disulfide bonds in the CH3 domain were the least susceptible to reduction (Liu, H, et al Anal. Chem., 2010, 82, 5219-5226).
Based on the more susceptibility of inter chain disulfide bonds in human IgG1 antibodies, several institutions and companies adopted the chemically specific conjugation strategy through rebridging reduced interchain disulfide bonds of a native antibody, such as, using bromo or dibromo-maleimides, called next generation maleimides (NGMs) (Schumacher, F. F., et al 2014, Org. Biomol. Chem. 12, 7261-7269; UCL Cancer Inst.), applying bis-alkylating reagents via a three-carbon bridge (Badescu, G., et al., 2014, Bioconjug. Chem. 25, 1124-1136., WO2013/190272, WO2014/064424 for PolyTherics Ltd), with di-substituted heteroaryl bridge (US Pat Appl. 2015/0105539 for Concortis Biosystem), or through di-maleimide as a bridge (WO2014/114207). We have also used bromo maleimide and dibromomaleimide linkers to conjugate both drugs and antibodies for a quite while (WO2014/009774, PCT/IB2012/053554). However, these above bridge linkers were designed in the way to conjugate only one cytotoxic agent to a pair of disulfide bonds, and therefore at most of time they only produced ADCs at DAR less than 2 (drugs per antibody), due to limited numbers (about only two pairs) of reduced disulfide bonds are more accessible for conjugation.
As one of the major issues for ADCs is the limited numbers or amount of cytotoxic compounds that ultimately reaches the tumor, and the favorable DAR over 3 is much important factor for improvement of ADC therapeutical index (Epenetos, A. A. et al, Cancer Res., 1986, 46, 3183-3191; Chari, R. V. Acc. Chem. Res., 2008, 41, 98-107, Zhao, R. Y. et al, 2011, J. Med. Chem. 54, 3606-3623), we therefore disclose novel disulfide bridge linkers of this invention that are able to conjugate two or more drugs per linker for achieving higher DARs (≥4) via re-bridging the susceptible a pair of thiols on the inter chain of IgG antibody that are reduced by TCEP and DTT. And the other reduced disulfide bonds that are inaccessibly reached by the bridge linkers, due to the big size of bridge linker containing two cytotoxic agents, can be recoupled (regenerated) by an oxide, e.g. dehydroascorbic acid (DHAA) or Cu (II), at the end of conjugation. In a word, these bridge linkers of the invention can make homogeneous production of specific ADCs in a simple way.